The broad objective of the proposed research is to examine the functional adaptation of the heart and circulation to the demands of exercise and other forms of stress in trained conscious dogs. The areas of particular interest are: a) the respective contributions of neural, humoral and intrinsic myocardial factors to the cardiac response to an increase in volume or in pressure load, b) the reflex integration of the response of the heart with that of the peripheral vasculature so as to obtain the requisite distribution of blood flow and maintenance of systemic blood pressure, and c) the degrees to which the heart or the peripheral circulation can each accommodate for a disturbance in the integrated response of the other system. The general method of approach will be the measurement of systemic and regional blood flow, perfusion pressure and blood volume in conscious dogs before and after 1) interruption of the vasomotor nerve supply to a particular organ or organs (i.e. heart, skeletal muscle, splanchnic area), as well as to the total animal, 2) chemical blockade of the cardiovascular actions of circulating catecholamines, and 3) interruption of afferent nerve impulses from carotid and aortic press-receptor areas. Graded excercise will be the particular means of stressing the cardiovascular system, but excitement and drug induced hypo and hypertension will be also employed.